Diffusion bonding apparatus

ABSTRACT

Apparatus for isostatic pressing, diffusion bonding, and the like, having a furnace designed to prevent turbulence in the hot zone so substantially the entire free space may be occupied by the work. In one form, the furnace can operate in vacuum to 100 microns and in air or inert gas at temperatures to 2,600* F. and pressures to 30,000 pounds per square inch which shortens the operating cycle by substantially eliminating the heatup and cooldown time.

United States Patent Inventor Appl. No.

Filed Patented Assignee Charles W. Smith, Jr. Fairview, Pa.

Apr. 15, 1970 Jan. 4, 1972 Autoclave Engineers, Inc. Erie, Pa.

DIFFUSION BONDING APPARATUS 8 Claims, 9 Drawing Figs.

US. Cl 219/85, 13/22 Int. Cl. 823k 11/04 Field of Search 219/85, 22, 25

[56] References Cited UNITED STATES PATENTS 3,150,226 9/1964 Thorne eta]. 13/25 3,301,541 1/1967 lpsen 13/31 X Primary Examiner-J. V. TruheAssistant Examiner-J. G. Smith AttorneyRalph Hammar ABSTRACT: Apparatusfor isostatic pressing, diffusion bonding, and the like, having afurnace designed to prevent turbulence in the hot zone so substantiallythe entire free space may be occupied by the work. in one form, thefurnace can operate in vacuum to 100 microns and in air or inert gas attemperatures to 2,600" F. and pressures to 30,000 pounds per square inchwhich shortens the operating cycle by substantially eliminating theheatup and cooidown time.

msmim 4m 3.832.954

SHEET 1 OF 3 FIG./ 5

PATENTEUJAN 41972 3.832.954

' sum 2 OF 3 IN V ENTOR.

mtmam 41912 36321954 SHEET 3 OF 3 IN V EN TOR.

DIFFUSION BONDING APPARATUS This invention is intended to preventturbulence in the hot zone of diffusion bonding apparatus sosubstantially the entire free space may be used as a working zone.

In the drawing, FIG. 1 is a sectional elevation of diffusion bondingapparatus, FIG. 2 is a section on line 2-2 of FIG. 1, FIG. 3 is anelevation of one of the heating elements, FIG. 4 is a section on line 44of FIG. 3, FIG. 5 is a diagram of the heating input, FIG. 6 is a diagramof the temperature at the hot zone, FIG. 7 is a plan view of the furnacesupport, FIG. 8 is a control diagram, and FIG. 9 is an elevation of thebottom loaded apparatus.

The apparatus has a pressure vessel body I surrounded by a water jacket2 which cools the walls of the vessel. The lower end of the body isclosed by an annular nut 3 and the upper end of the body is closed by aconical nut 4 which is adapted to quick opening and closing. The body 1is supported on a stand 5 The upper end of the pressure vessel is sealedby O-rings 6 and 7 carried by a seal ring 8 and respectively makingsealing engagement with the bottom surface 9 of the closure 4 and withthe cylindrical sidewall 10 of the body 1'. The lower end of thepressure vessel is sealed by a plug 11 having a head 12 slidablyreceived within a bore 13 at the lower end of the body and carrying anO-ring 14 making sealing engagement with the bore. A locknut l5 threadedon the stem of the plug 11 clamps a shoulder 16 on the underside of thehead solidly against the upper end of the nut 3. By removing the locknut15, the plug 11 may be lifted out of the nut 3.

The furnace in which the diffusion bonding takes place is mounted on asupport 17 fixed to the head 12 by screws 18. As shown in FIG. 7, thesupport 17 has an annular opening supporting ring 19 carrying studs 20which carry supporting and guide plates 21a, 21b, 21c, 21d for thefurnace heating elements and for the associated electrical terminalconnections. The supporting plates and the terminal connections are inthe cool zone of the furnace which comprises all of the furnace belowthe supporting plate 22. The supporting plate 22 is partially supportedby nuts 23 on the upper ends of the studs 20 and is further supported bynuts 24 on the studs 25 carried by the supporting ring 19. Thesupporting plate 22 carries a refractory disc 26 which serves as thebottom wall of the hot zone and which also supports refractory cylinders27 and 28 serving as the sidewall of the hot zone. At the top of the hotzone of the furnace is an annular guide 29 of refractory material and atop supporting structure 30 closed by a cover 31 which may be lifted byan eye bolt 32.

The heating elements are of two kinds. There is a set of angularlydistributed heating element 33 which have their active heating surfaceextending substantially the full distance between the disc 26 and thetop guide ring 29. Theseheating elements extend through a middle guidering 34 between the cylindrical sections 27 and 28. The purpose of theheating elements 33 is to supply heat throughout the entire hot zone ofthe furnace. The other set of heating elements 35 is similarlydistributed around the inner circumference of the furnace and each hasits active heating length extending from the disc 26 about half way tothe top of the furnace. The purpose of the heating elements 35 is tosupply heat to the lower part of the hot zone. The heating elements arewithin the hot zone near its outer periphery and are spaced from thewalls of the hot zone to prevent heat transfer by conduction. Each ofthe heating elements 33 and 35 is of the general structure shown inFIGS. 3 and 4 and comprises a hollow tube of silicon carbide which issplit at 36 at its lower end and each side 37, 38 of the split isconnected by spiral sections 37a and 38a to the continuous upper end 39of the heating element. The active heating sections of the heatingelements are the spiral sections 37a and 38a. The split lower ends areconnected to power terminals 40 and 41 in the cool zone of the furnace.The actual connections to the heating elements are not shown but arediagrammatically indicated since these are well known.

FIG. 5 shows the maximum heating power output of the heating elements 33and 35. The power output of the heating elements 33 is indicated at 42and the power output of the heating elements 35 is indicated at 43.

The sets of heating elements 33 and 35 are separately controlled. Thecontrol for heating elements 33 has a thermocouple 45 which senses thetemperature in the upper part of the hot zone of the furnace. Thecontrol for the heating element 35 has a thermocouple 46 which senses tothe temperature in the lower part of the hot zone of the furnace. Thethennocouple 45 is connected to lead wires 47 which extend up throughthe guide ring 29 and top supporting structure 30 and down throughthermal insulation 48 surrounding the sidewalls 27, 28 of the furnace.The thermocouple 46 is connected to a lead wire 49 which also extends upthrough the guide ring 29 and top supporting structure 30 and downthrough ceramic thermal insulation 48, e.g. zirconia, surrounded bymetal shell 48a. Both of the thermocouple lead wires 47 and 49 extendout through the plug 11 in the bottom closure 3 of the pressure vessel.The electrical power input lines (not shown) also extend through theplug 11 and are connected to a bus structure 50, only part of which isshown. The furnace structure is all mounted on the plug 11 which can beassembled and tested prior to insertion in the pressure vessel.

The furnace is surrounded by a metal seal can 51 closed at its top andopen at its bottom, which telescopes over substantially the full lengthof the furnace. The top wall 52 of the seal can is provided with alifting ring 53. The wall 52 is protected from the furnace heat byfibrous thermal insulation 54. The sidewalls 55 of the seal can areadjacent the inner sidewalls of the pressure vessel, are cooled by thecooling jacket 2 and are also protected from the furnace heat by theinsulation 48. The lower end 56 of the seal can is protected from thefurnace heat by insulation 57, 58 in the spaces above and below thespacer plates 21a, 21b, 21c, 21d.

The operation of the furnace will be explained in connection with thecontrol diagram of FIG. 8. At the start of the operation, the workpiece59 consisting of several parts to be difiusion bonded is placed on thebottom wall 26 of the furnace hot zone, the cover 32 and seal can 51 areinstalled on the furnace and the pressure vessel is closed by its topclosure 4. If desired, the interior of the pressure vessel is thenevacuated through vacuum line 60. When the desired vacuum is reached,the valve 61 is closed. Any of the usual gases is now supplied underpressure through line 62 and the heating elements 33 and 35 are turnedon, each heating element being controlled by a separate control 63, 65.At the end of several hours, the furnace will come up to its operatingtemperature, for example, 2,600? F and the parts of the workpiece 59will be fusion bonded in a few minutes. At this point, the inert gas iswithdrawn through line 62 to depressurize the furnace, the pressurevessel cover 4, the seal can 51 and furnace cover 32 are removedexposing the open top of the furnace so that the finished workpiece 59can be lifted out and a new workpiece substituted. When the furnace isopened, the workpiece and the heating elements 33, 35 are exposed to airbut are not oxidized because they are of silicon carbide. The heatingelements and the part of the furnace do not substantially cool downwhile the completed workpiece 59 is being removed and another workpiecesubstituted. There is no substantial change in the fumace hot zonetemperature which remains substantially as indicated by line 66 in FIG.6. It will be noted that the hot zone temperature remains substantiallyconstant from top to bottom so that convection cannot significantlychange the temperature to which the workpiece is being subjected. Thetime between cycles is that required for handling of the parts whichmust be removed and replaced. There is no need to wait for the furnaceto cool down between cycles. This results in significant shortening ofthe average process time for the bonding operation. For example, in aprior furnace which could not be operated in vacuum, air and inert gas,it might take 3 hours for the furnace to heat up and 3 hours for thefurnace to cool down.

When starting cold, both sets ofheating elements are on and transferheat by radiation and convection to all portions of the hot zone. Thetop, side and bottom walls of the hot zone become hot. As the furnacetemperature increases, the demand for heat from the elements 33decreases and at operating temperature little or no heat is requiredfrom the elements 33 and the heating elements 35 maintain the furnacetemperature which remains substantially constant from the top to thebottom of the hot zone. This is possible because turbulence in the gasis avoided. Under operating conditions, gas under pressure permeates theentire interior of the pressure vessel, with the coldest gas adjacentthe water-cooled pressure vessel body 1 and the hottest gas within thehot zone. Within the hot zone, convection is unobstructed so gasadjacent the heating elements is free to flow both toward the outerwalls of the hot zone and toward the workpiece. However convection fromthe hot zone to the cooler gas outside the walls of the hot zone iseffectively blocked by means in the form of restricted passagewaysprovided by the seal can 51, thermal insulation 48, 54, 57, 58, whichcauses the flow of gas between the hot and cold zones to take place bydiffusion rather than by ordinary hydrodynamic flow. By avoidingturbulence in the hot zone, no measurable difference in temperature inthe workpiece can be measured from bottom to top. There should be a freespace for convection currents within the hot zone so the temperature ofthe gas can equalize. Direct contact with the heating elements isavoided. The workpiece may occupy substantially all of the free space atthe center of the hot zone so long as there is space for freecirculation of gas about the workpiece and contact of the heatingelements with the work and with cooler walls is avoided. By way ofexample, in a furnace having a hot zone approximately inches in diameterand 20 inches high, the work can be 6 inches in diameter and from 18 to19 inches high and the clearance between the heating elements and thework and between the heating elements and the sidewalls of the hot zonecan be from one-fourth to one-half inch. I

FIG. 9 shows the apparatus of FIG. 1 modified so that it may be bottomloaded. The changes necessary for bottom loading are (1) enlarge thelower end of the body of the pressure vessel as shown at 67 so that theseal can and the enclosed apparatus may be installed from the bottom,(2) change the plug on which the apparatus is supported from cylindricalto annular shape as shown at 1 la, and (3) provide an insulating piston68 for carrying the work. The power and control connections will now bemounted on the annular plug 110. Appropriate seals 69 and 70 areprovided between the plug 11a and the lower end of the pressure vesselbody and between the insulating piston 68 and the plug 11a. When thesechanges are made, the apparatus can be used for either top or bottomloading.

The silicon carbide heating elements 33, 35 are advantageous when theapparatus is operated so as to expose the hot elements to air. Otherheating elements which will not stand exposure to air while hot may beused if the apparatus is cooled down between cycles or if the heatingelements are protected by an inert or nonreactive gas such as argon, bycontinuous purging with the gas when the apparatus is open.

The furnace is intended for operation at pressures up to an operatingtemperature of 2,450 F.

What is claimed is:

1. Apparatus for diffusion bonding and the like comprising an uprightpressure vessel having a filling of inert gas under pressure requiredfor diffusion bonding, said vessel having top, bottom and sidewalls,means for cooling the pressure vessel walls while holding gas under saidpressure, a furnace having a hot zone surrounded by refractory top,bottom and side walls each spaced from the corresponding part of thepressure vessel, a plurality of heating elements within the hot zonedistributed to leave an open space at the center of the hot zone, meansfor supporting work in said open space out of contact with said heatingelements and providing space for free circulation of said gas about thework, said heating elements being out of contact with each other andsaid furnace walls and spaced to permit free convection from the heatingelements to the furnace walls and to said open space, the gas underdiffusion bonding pressure permeating the entire interior of thepressure vessel with the coldest gas adjacent the cooled pressure vesselwalls and the hottest gas within the hot zone, means in the form ofrestricted passageways for blocking convection of gas under diffusionbonding pressure from the hot zone to the coldest gas adjacent thecooled pressure vessel walls and limiting the flow of said gas from thehot and cold zones to diffusion rather than by ordinary hydrodynamicflow and thereby avoiding turbulence in the hot zone.

2. The apparatus of claim 1 in which said means for blocking convectionof said gas from the hot zone comprises a seal can open at the bottomand closed at the top and sides and telescoped over the furnace betweenthe furnace and the pressure vessel and thermal insulation between thetop and sides of the furnace and the top and sides of the seal can andbetween the bottom wall of the hot zone of the furnace and the sides ofthe seal can and the bottom wall of the pressure vessel.

3. The apparatus of claim 1 in which the heating elements comprise aplurality of sets and one set of heating elements is wholly in the lowerpart of the hot zone.

4. The apparatus of claim 1 in which the pressure vessel has a removablebottom closure and the furnace is mounted on said removable bottomclosure.

5. The apparatus of claim 1 in which the pressure vessel has an annularbottom closure with a removable closure at the center of the annularclosure and the furnace is mounted on the annular closure.

6. The apparatus of claim 5 in which an insulated piston having meansfor carrying a workpiece on its upper end extends from said removablebottom closure up through the bottom wall of the furnace.

7. The apparatus of claim 3 in which another set of heating elements hasa height substantially equal to the full height of the hot zone.

8. The apparatus of claim 7 having separate control means for each setof heating elements.

1. Apparatus for diffusion bonding and the like comprising an uprightpressure vessel having a filling of inert gas under pressure requiredfor diffusion bonding, said vessel having top, bottom and sidewalls,means for cooling the pressure vessel walls while holding gas under saidpressure, a furnace having a hot zone surrounded by refractory top,bottom and side walls each spaced from the corresponding part of thepressure vessel, a plurality of heating elements within the hot zonedistributed to leave an open space at the center of the hot zone, meansfor supporting work in said open space out of contact with said heatingelements and providing space for free circulation of said gas about thework, said heating elements being out of contact with each other andsaid furnace walls and spaced to permit free convection from the heatingelements to the furnace walls and to said open space, the gas underdiffusion bonding pressure permeating the entire interior of thepressure vessel with the coldest gas adjacent the cooled pressure vesselwalls and the hottest gas within the hot zone, means in the form ofrestricted passageways for blocking convection of gas under diffusionbonding pressure from the hot zone to the coldest gas adjacent thecooled pressure vessel walls and limiting the flow of said gas from thehot and cold zones to diffusion rather than by ordinary hydrodynamicflow and thereby avoiding turbulence in the hot zone.
 2. The apparatusof claim 1 in which said means for blocking convection of said gas fromthe hot zone comprises a seal can open at the bottom and closed at thetop and sides and telescoped over the furnace between the furnace andthe pressure vessel and thermal insulation between the top and sides ofthe furnace and the top and sides of the seal can and between the bottomwall of the hot zone of the furnace and the sides of the seal can andthe bottom wall of the pressure vessel.
 3. The apparatus of claim 1 inwhich the heating elements comprise a plurality of sets and one set ofheating elements is wholly in the lower part of the hot zone.
 4. Theapparatus of claim 1 in which the pressure vessel has a removable bottomclosure and the furnace is mounted on said removable bottom closure. 5.The apparatus of claim 1 in which the pressure vessel has an annularbottom closure with a removable closure at the center of the annularclosure and the furnace is mounted on the annular closure.
 6. Theapparatus of claim 5 in which an insulated piston having means forcarrying a workpiece on its upper end extends from said removable bottomclosure up through the bottom wall of the furnace.
 7. The apparatus ofclaim 3 in which another set of heating elements has A heightsubstantially equal to the full height of the hot zone.
 8. The apparatusof claim 7 having separate control means for each set of heatingelements.